Although a cemented carbide alloy has conventionally been used as a writing ball for a ballpoint pen, a writing ball made of ceramics has also been used in recent years. The writing ball made of ceramics is less susceptible to corrosion caused by ink and has a feature that there is little wear to a ball receiving seat. For example, JP 59-135195 A discloses a writing ball made of zirconia sintered body ceramics. Moreover, JP 2001-80260 A discloses a writing ball made of a zirconia sintered body, as one of writing balls suitable for ink containing hard inorganic pigment.
In the writing ball described in JP 2001-80260 A, spaces between hard grains composing the writing ball are much narrower if compared with a conventional cemented carbide alloy ball. Therefore, since the fragments of the hard grains exposed on the surface of binder metal combining the hard grains are minute and since the number of the fragments is very small, it is less likely that the fragments will drop from the surface of the writing ball and act as abrasives during writing, preventing the wear of a ball receiving seat. Moreover, since the distances between boundaries of the hard grains are narrow, the edges of the hard grains of the boundaries are less likely to act as cutting blades, preventing wear to a ball receiving seat.